That means the shortcut to RoR.exe is no longer valid, and it can't find it.
Usually happens when antivirus software removes an .exe.
Go to C:\Program Files\Rigs of Rods
and see if RoR.exe is in there, run it directly if so, make sure it works, and create a new shortcut.
If not, check whether your antivirus ate it (shouldn't have), and restore it. How to do this will depend on the software you use.
As a last resort you can just re-install RoR, but the antivirus will probably eat the .exe unless you add an exception.
There could be a different reason why the .exe is missing, or the shortcut is invalid. I've got no other suggestions though, unless you installed it on a removable drive and the drive letter changed.