[Koha] New Member and Problem

Dear All, I'm a new member, and having a major problem with Koha. We have imported all our MARC records having converted them from an Excel spreadsheet. This worked, but the Call Number did not show. So we un-imported the records, made some adjustments and tried again. Now we cannot search by Title for any record, as we get the error message "Can't call method "as_usmarc" on an undefined value at /usr/share/koha/lib/C4/Search.pm line 2157". I've read that this means the records need reindexing. We just can't figure out how to do this . . . . Failing this, how can we delete ALL the records and start again? Unimporting seems to take about 24 hours, and we just don't have time. Please help!! Thanks, GT