Diagnosis
The control board has likely locked up or frozen, which can happen after a power surge or a software glitch. It's essentially a small computer that needs a hard reset.
How to Fix It
Don't just turn it off at the unit's power button. You need to completely cut power. 1) Locate the power cord where it plugs into the wall. 2) Unplug it. 3) Leave it unplugged for at least five full minutes. This drains any residual power and lets the control board reset. After five minutes, plug it firmly back into the wall. The display should light up normally. This fixes the vast majority of touchscreen freezes.
If That Didn't Work
Check for any visible damage or moisture on the touchscreen itself. If it was cleaned with a very wet cloth, liquid might have seeped in. If the reset didn't work and the screen is dry, the control panel itself may have failed and will require replacement.
Heads Up
When cleaning the touchscreen, always spray your cleaner onto a microfiber cloth first, never directly onto the glass. Excess moisture running down the front can get inside and short the sensitive electronics behind the panel.