One method is to value the company at four times the annual net profit. That is based on an assumption that the investment is higher risk than your ordinary stock market return.
That also assumes a passive investment. The buyer should base the numbers on paying employees to do all the work (including accounting and auditing).
Using this model, if the business cannot support itself (business is losing money) it has NO value.
I don't disagree...but someone isn't going to be investing any money in the business, they are valuing it for the purposes of a property settlement in a divorce.
Another method is two times annual sales.
Another method is to simply value fixed assets.
One major point however to consider is, if this is the type of business that relies strictly on the work of the business owner (a consultancy, a plumber operating on his own, a doctor or lawyer with their own office, etc) then really, the only value considered should be the value of fixed assets.