To the extent understood, California Civil Code section 3289 states:
(a) Any legal rate of interest stipulated by a contract
remains chargeable after a breach thereof, as before, until the
contract is superseded by a verdict or other new obligation.
(b) If a contract entered into after January 1, 1986, does not
stipulate a legal rate of interest, the obligation shall bear
interest at a rate of 10 percent per annum after a breach.
For the purposes of this subdivision, the term contract shall not
include a note secured by a deed of trust on real property.
In a nutshell, you can normally charge 10 percent interest if no interest rate was agreed upon. If have a continuing issue, you should contact a attorney for detailed advice.