Credit can go from good to bad to poor for a number of reasons, including missed payments, late payments, etc. On the other hand, bad credit can improve to good credit, too. But this takes a little bit of work.

Depending on your situation, debt consolidation and/or credit repair may be the route to take. Also, certain credit cards are made to help "rebuild" credit histories. Secured credit cards are for people with no credit or poor credit who are trying to build or rebuild credit history. But some unsecured credit cards can also serve similar purposes.

Often with cards that help to rebuild credit, low credit lines are given ($250 or so) and additional fees may apply (application fees, etc.). Be sure to read over any terms and conditions for any of these services before applying. Be certain of any fees that you may incur before proceeding. But if you use the card responsibly and pay all of your bills on time, you can ask for a credit increase down the road. The extra fees and low credit lines will be worth it if it helps get your overall credit back on track.