[meow]

It could be happening for multiple reasons. The most common are

- Your battery on the computer is weak and the BIOS is losing settings. You can usually tell if it's this if the date and time are incorrect on the BIOS page(s).
- Some (or all) of the boot devices can't be found
- Boot order is messed up

Google is probably your best friend in this case.
Try doing a quiet boot and see what happens.