Hi Teesside,
Welcome to the forum.
Could be a number of things: overheating, corrupted hard drive, box needs resetting.
Have you unplugged box or reset it? Try a soft reset 1st. Press power button for 8 seconds till box reboots. If no success after this, try unplugging for saying 15/30 minutes and see if this fixes problem. If not, do a factory reset, this will keep recordings, but make a note of schedules and also app passwords will need rekeying in after reset.
Hopefully one of these tips works. If not, reformat hard drive (will lose recordings) if still problem it’s either faulty hard drive or hardware issue with overheating. Please let forum know how you get on/ if you need more help. John L