First, see if your problem is discussed in detail either on this FAQ, on Codemasters' official OFP FAQ or on BIS's troubleshooting FAQ.
Next, send an email to Codemasters' customer service. Unlike many other PC game companies, Codemasters will usually respond within 24 to 48 hours. Make sure you provide a detailed explanation of the problem and an accurate description of your system's hardware and software specifications. Be clear and don't mince words.
At the same time, it doesn't hurt to search the forums for existing threads from someone who may have already experienced the same or a similar problem.
If you cannot find anything on this FAQ, on Codemasters' official OFP FAQ or by searching the forums, and Codemasters' customer service hasn't resolved the problem, then post a new thread on the forum of your choice and explain that you've been through all the loops but still have come up empty-handed. This ought to generate the minimal amount of pity required for a reply from someone with a reasonable response.