1. Ensure that your phone is actually powering off, rather than just going into sleep mode. If the lock screen displays right away, the phone is going into sleep mode. 
2. If any new apps were installed recently, see My apps are not working for further assistance. 
3. Verify that the battery indicator shows more than 10% power. If it shows less, you must charge your phone.
   
   NOTE: If battery power is low or the phone will not turn back on, see I am having charging problems.
   

Follow these additional steps for HTC phones with a removable battery:

1. Open the back of your phone and check if the battery is loose.  
2. Remove the battery and inspect it for any damage, such as:
   
   Swelling or warping
   Missing contacts
   Dents or scratches
   
   NOTE: If the battery shows damage, you need a new battery. Third-party chargers not designed for the phone may cause swelling and damage the battery.  
3. If you have another battery for the phone, replace the current battery and see if the issue persists.