iPad Air screen too dark

Have you tried adjusting the brightness?

In Settings app, Display, try turnimg OFF, then ON again Auto Brightness.

Try a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

Good Luck!

If changing the brightness and resetting didn’t help, it is time to visit in Apple Store or authorized Apple service provider. Demonstrate the problem and let them assess it and tell you how to proceed.

Change brightness and/or True Tone settings at Settings > Display & Brightness.