Screen is flashing 0.00

I had the same problem with my HP 11C. The following solution worked for me.
1. Remove the battery cover.
2. With the batteries in place (i.e. DO NO REMOVE THEM), use a small piece of wire (I used a standard staple bent at one end), short the terminals FOR A BRIEF MOMENT between both ends of the set of batteries (the HP-11C has three). This means somehow inserting the staple such that the contacts in the batteries receptacle are shorted.
3. The flashing will stop. Then push the ON button until the calculator turns on. If it does not work the first time, remove the batteries and resinsert them.
4. You may have to repeat this procedure. I had to do it twice before my calculator worked again.