Why does the program assemble, but do nothing?

Why does the program assemble, but do nothing?

Here is a copy of the source:
extrn MessageBoxA: PROC
extrn ExitProcess: PROC
.data
mytit db 'The 64-bit world of Windows & assembler...', 0
mymsg db 'Hello World!', 0
.code
main proc
mov r9d, 0 ; uType = MB_OK
lea r8, mytit ; LPCSTR lpCaption
lea rdx, mymsg ; LPCSTR lpText
mov rcx, 0 ; hWnd = HWND_DESKTOP
call MessageBoxA
mov ecx, eax ; uExitCode = MessageBox(...)
call ExitProcess
main endp
End
Right now, I am just trying to get my 'first' x64 assembly program up and
running so that I can start playing around and actually learn some
assembly, so I just literally copied this source from here in an attempt
to see if I could get anything to assemble correctly, but so far, no luck.
If I assemble this, I don't get any errors, neither during assembly-time
nor run-time, but the program doesn't run as it's supposed to: it doesn't
appear to do anything at all. Once the executable has been generated and I
double-click on it, nothing happens, and if I go to the task manager, it
doesn't seem to be running in the background either. What's going on?
I am using the default settings for MASM64 that are generated upon
checking "masm" under "Build Customizations..." (found by right-clicking
the project in the Solution Explorer) and changing the platform from Win32
to x64 in the Configuation Manager, the exceptions being that I've changed
the "Entry Point" linker option to "main" and the "SubSystem" linker
option to "Windows". (All of this is done in Visual Studio 2012.)
The debug info generated by running the program in Visual Studio is
'Hello World (ASM).exe' (Win32): Loaded 'D:\Google Drive\My
Documents\Visual Studio
2012\Projects\Hello World (ASM)\x64\Release\Hello World (ASM).exe'.
Symbols loaded.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\ntdll.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\kernel32.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\KernelBase.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\user32.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\gdi32.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\imm32.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\msctf.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\msvcrt.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\nvinitx.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\advapi32.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\sechost.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\rpcrt4.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Program Files\NVIDIA
Corporation\coprocmanager\detoured.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Program Files\NVIDIA
Corporation\coprocmanager\Nvd3d9wrapx.dll'. Cannot find or open the PDB
file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\setupapi.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\cfgmgr32.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\devobj.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Program Files\NVIDIA
Corporation\coprocmanager\nvdxgiwrapx.dll'. Cannot find or open the PDB
file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\PROGRA~1\NVIDIA~1\NVSTRE~1\rxinput.dll'. Module was built without
symbols.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\ole32.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded
'C:\Windows\System32\oleaut32.dll'. Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Loaded 'C:\Windows\System32\combase.dll'.
Cannot find or open the PDB file.
'Hello World (ASM).exe' (Win32): Unloaded
'C:\PROGRA~1\NVIDIA~1\NVSTRE~1\rxinput.dll'
'Hello World (ASM).exe' (Win32): Unloaded 'C:\Windows\System32\combase.dll'
'Hello World (ASM).exe' (Win32): Unloaded 'C:\Windows\System32\oleaut32.dll'
'Hello World (ASM).exe' (Win32): Unloaded 'C:\Windows\System32\ole32.dll'
First-chance exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
Unhandled exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
First-chance exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
Unhandled exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
First-chance exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
Unhandled exception at 0x000007FA9A09B9FB (gdi32.dll) in Hello World
(ASM).exe: 0xC0000005: Access violation reading location
0xFFFFFFFFFFFFFFFF.
The program '[1080] Hello World (ASM).exe' has exited with code 0 (0x0).