While you are recuperating from an injury, you are entitled to be compensated for time lost from your regular occupation. Even if you are not employed, you may be entitled to the reasonable value of the time lost if your injury prevented employment and you had a reasonable expectation of being employed at the time of the accident.
In most cases, if you are employed, it won't be hard to prove your lost wages. You are entitled by law to receive an amount equal to your regular pay (without deduction for taxes) for each day you were unable to work. If you utilize sick days or vacation days to recuperate from your injuries, you are still entitled to be paid for the sick days or vacation time you no longer have.
The insurance company will require that your doctor approve each day you were off work following the collision. Make sure you have an off-work slip from your doctor each day you missed so it can be documented. Otherwise, the insurance company may not accept it.
If you are self-employed, or if your income fluctuates, you will probably have a more difficult time proving your lost earnings. The insurance company may dispute estimates based on historical records - and it may be too expensive to prepare an audit. If you have incurred permanent injuries, you may have the right to receive payment for loss of ability to earn in the future. Establishing such amounts is very difficult and usually requires the services of an economist as an expert witness and a vocational rehabilitation expert to determine your work future. Again, this is a time when professional legal counsel is invaluable!