My iPhone is on but it won't let me use any apps and the power button is not working

See this if you cannot turn the iPhone on. If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support It describes how to force restart the iPhone. When you say that the power button will not work, has it been damaged, or is it just that the iPhone is frozen that you cannot turn it on?

Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.