Hold down the "shift" key on your keyboard and open the database. You must keep it held untill the DB opens. This should let you into the design part of access. Hope it helps.
Try copying the DB into another folder and opening it. sometimes access crashes and leaves the db locked. Let me know if this solves your problem. If not give me more details of the error message.
Ok, here's a quick nasty way that works. Create a table with a single field in it called length. In the field properties change the size to 8. Link your form to the table and the text box to the length field. You should now only be able to enter 8 characters max into that text box. Let me know...