Model starts printing above bed

I've printed a couple dozen items on my Anycubic Kossel Plus successfully. Today I tried printing a file and the extruder started laying down filament about 2" above the bed. The Cura model looked like it was sitting on the bed. I rebooted and tried a file I'd done before and it printed normally. Both files were sliced with Cura 4.2.1. What can I check to make sure the file begins on the bed?

you need to calibrate your z axis.

What firmnware are you using.

I will do the calibration but why would some prints work normally and others start high?
The firmware is what came from the factory.

could also be the slicer
Some models aren't always flat on the 'table' before sliging.
Could also be a dodgy sd card.