The remote to my 2002 cadillac deville dont unlock or lock my doors or open my trunk like it used to. my power locks dont work. i got in the car last week and on car display says theft attempt, the lights were flashing horn blowing. the remote haven't work since

Either via an actual theft attempt or you (or someone with your keys) not realizing that you/they actually armed the vehicle and opened the door or trunk afterwards.

Essentially this sounds like the vehicle was entered after being armed and the proper precedures not met to reset it.

The car may just need to be locked from the outside with the key (not the remote or keyless entry) while the doors are closed and windows are up, and then also unlocked with the key...

However it sounds as though you have perhaps driven it since then. If this is the case

You may want to check all your fuses and replace the ones affected by the anti theft system.
Security - Pwr Locks - Horn - Pwr Trunk Release _ Int Lights - and maybe The Emergency Flasher as well.

Be sure and check under the Dash, Hood and under the Back seat.

Cadillac recommends after a theft attempt, the vehicle may need to be serviced. So I recommend going through the fuses first, even replacing the battery in your remote fob and then if the problems still persist. have the vehicle serviced.

Good Luck and Happy Motoring

Respectfully