Sounds like you are booting into single-user mode. Normally, you get there by holding down apple-s during bootup. But you also get there if your OS install is damaged and it can't do the later bootup stages.
Try a disk repair, a permissions repair, or an OS archive reinstall. In that order. The first two are painless, the OS reinstall will no doubt mess up a few things, requiring preferences to be reset and perhaps some applications to be reinstalled.