Transmission won't shift on 1998 Chevrolet Cavalier

My car has an automatic transmission but as of last night it will not shift out of first gear what make it do this and how could I fix this .Thank-you

2 answers 3 comments
First check the transmission fluid level, look for signs of oil leaks from the transmission area. Next try manually shifting the transmission with the shift lever after that the problem is probably internal in the transmission but is best checked out by a shop familiar with Chevy or transmissions.
could it be the valve body
its the shift solenoid
this is not the answer we need. I need to change the shift solonoids and want to know the best way to do it myself. Do I need to remove the tranny from the car or what?
transmission wont shift