Err.. before hitting the thing with recovery software & stuff...
STOOOOOPID QUESTIONS.... Lets eliminate the possible 'silly' errors or mistakes before getting into a panic and presuming major sytems failure, eliminate common human error!
1/ How are you viewing the card contents?
a) from camera
b) from card-reader
If from camera... is it the SAME camera as took pics? If not, could be that the camera isn't recognising the file-format from another camera.
2/ What file / photo management program are you using to look at your card?
eg: Windows Explorer; Adobe Light-Room; Kodak Easy-Share; Nikon View NX etc
3/ Are you SURE 'last nights pics' are actually on that card?
You don't say what camera its from. Does camera you took last nights pictures on have any internal memory? Done this one many times; took card out to find no 'photos only to find that card wasn't 'clicked in' and camera was writing to internal, not card.
4/ Have you done anything 'different' from taking to viewing last nights 'set'?