As a general rule, if you’ve never filed taxes for your dormant LLC, you do not need to file taxes as long as you haven’t been using the LLC and it has had no revenue.
If it’s taxed as a disregarded entity, you only need to start doing Schedule C filings for it when there’s income that needs reporting or losses that you want to declare and apply/carry forward. Basically the same with partnership-type LLCs.
If it’s taxed as an S-Corp or C-Corp, however, you do need to do tax returns from the date the election was made effective.
Of course, the above assumes you haven’t been using the LLC as an operating entity and have generated no revenue. If you have, then you should talk with a tax attorney, CPA or accountant, and understand what your options are.