Really, I'd say just start over. If you haven't played for a long time, it's likely that a lot of the addons you were using haven't been updated. Rename your Interface and WTF folders, and then go find the download page for each addon you want to use now. Make sure it's been updated recently (within the last month or two) and then download a fresh copy. Then, before you log in, open the AddOns window from the character selection screen and check the "Load out of date addons" checkbox. Then, log in. If you get any errors, take a screenshot and post them.