Hello JJ, Try pressing the "Clear/OFF" button to stop the self cleaning cycle. It will take up to an hour before the oven will unlock the door, if the oven has started to heat up for the cleaning process
You may have started the default 3 hour self-clean cycle, so you can attempt to reduce the time to 2 hours, by pushing the timer down so it displays "2" which would indicate the shorter cleaning cycle. Then at the end of the timed cleaning cycle, it should open up, once the temperature has cooled sufficiently for it to be safe to open, again, which takes up to an hour to sufficiently cool.
Typically, when an oven is accidently put into self clean mode, the timer "kicks in" and the timer has to run out before the oven will unlock. With the older mechanical knob timers, you could just rotater the timer until you reached the end of the pre-set time. With the electronic timers, you have to let the timer "count down" until it reaches zero before the oven will unlock the oven door.
Here's a link to the Frigidare site where you can select your specific FEF352 version and download a owners manual, free of charge.
Just enter in FEF352 and then scroll down the page until you find your particular version of teh FEF352 oven/range and then click on "complete owners guide" (english) to get the manual in an Adobe pdf format.
I hope you find this Very Helpful and best regards!