Security code (similar to a captcha, but not) just stopped working on my checkout page

I have a cart on my site that has been there for years. I can't even remember how long it's been since it's had any changes made to it, pretty long, which is why it kinda surprised me when the security feature just stopped working on my checkout page, because I hadn't made any changes so it's weird that it just broke all by itself. I am looking for someone to fix it asap. I did billing today (as it's the 1st, LOL!) and I need for my clients to be able to go in and pay their bill online, but they can't because you can't get past the security code problem. If you add anything to the cart then go to checkout, you can see on the checkout page the security code feature is broken. I just need that fixed. I would imagine that it would take well under an hour for anyone who knows what they're doing to fix it. I need a fixed bid, and I know that there will be files you will want to look at before bidding (unless you're absolutely sure you know what's wrong and what needs to be done), so if you need to look at any files before placing a bid, please let me know. But no bids where you say oh $20 an hour and it could take 1-10 hours. Know what needs to be done and quote accordingly. I really do think it's a simple fix, just not simple enough for me to fix it myself :-(