The display window of a 301 may be blank or have an arbitrary number when powered up. When Nulling the display should briefly show 888 and then go to 0.00 so you can start measuring. If the display doesn't briefly show 888 or respond at all check to see if the unit is turned on, the lamp should be on whenever there is power.
The next step is to see if the Null switch or PCB are unplugged. 1) Unplug the instrument 2) Turn upsidedown and remove 4 rubber feet 3) Pull bottom cover off 4) Inspect connections to the Null switch and PCB, plug back in if needed.
If no problem is found contact Customer Service to have your 301 repaired.