Along the lines of what Doc Man is saying (and he's right), you need to make a backup at least two times a day, if not more. It's even better if that backup is to an external memory source (CD-Writer or USB Drive). I prefer a USB Drive because they're a lot less hassle, and you can get a 2GB USB drive for $17.99 here:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820134043
Basically, you want to backup around 10am, 1pm, 4pm, and whenever you stop for the night (6-8pm for me). Make a naming convention that's easy so that you can remember where you are. For example, today I'll have these files for the DB I'm writing:
DBName_2007-05-16a.zip
DBName_2007-05-16b.zip
DBName_2007-05-16c.zip
DBName_2007-05-16d.zip
Assuming I have no corruptions at the end of the day, I'll delete the a-c versions and remove the "d" from the last version. (Note that I use the format "_YYYY-mm-dd" at the end of the filename as that makes them sort correctly.) Then I keep each day's worth of changes for around 2-4 weeks, depending on how much progress is being made. Eventually the directory will look like this:
DBName_2007-05-07.zip
DBName_2007-05-08.zip
DBName_2007-05-09.zip
DBName_2007-05-10.zip
DBName_2007-05-11.zip
DBName_2007-05-12.zip
DBName_2007-05-13.zip
DBName_2007-05-14.zip
DBName_2007-05-15.zip
DBName_2007-05-16.zip
And so on. At a certain point, it no longer makes sense to maintain such a backup log -- you may have a different schedule than me; it's whatever you're comfortable with. Also, since USB drives are big and cheap now, space shouldn't be a concern when maintaining these backups. In other words, you could hold the last 50 DBs (a month and a half or so) on the key without worry.
The big, obvious advantage to this is avoiding what happened to you. It's saved me more than once in my years of work. Another, perhaps indirect advantage, is that the time it takes to compact/repair your DB, close Access, zip (or rar or whatever) the file, and copy it to the USB drive gives you a chance to stop staring at your project. Sometimes, a little distraction is a good thing. We've all been at the point that we've been staring at a problem for so long, the answer that is staring back at us is not obvious until we stop staring for a few minutes.
