this is exactly what happened to me before.

First, check w/ ur hr to see if they already inform INS about the termination of your work.
If NO, good, then discuss your situation with ur boss and ur hr, ask them to postpond to inform INS for 1 month (or whatever time both you feel confortable).
Before that time, you can:
1. change job, apply another H1B. This is the best option.
2. register as F1 in a community college. transfer your nonimmigration status.
3. apply B visa. (If you can not find any job or school,you may use this, but I wouldnt recommand this).

If hr already informed INS, check when,(exact which day they did it).
then, it's difficult. INS is very restrict on changing nonimmigration status after your previous status's got expired. (they rejected me for this reason)
The best way is to changing job and apply another H1B. As I understand, 3 weeks gap between old and new job should be acceptable for INS.

Try avoid to change to other catogary. But maybe taking action within 10 days should work. (Not postive)