A temporary fireplace seal will not prevent continued use of the fixture once the seal is removed. Before using it, however, you should make perfectly sure that the temporary flue plug or damper cover is entirely removed. Permanent seals can be removed with a lot more work. Assuming the fireplace is in the same condition as when it was sealed, you should be able to use it again.
Homeowners often temporarily plug a chimney flue or enclose the damper to stop cold air from coming in and warm air from leaking out. If you want to use the fireplace again, remove the insulation or foam board and make sure the damper opens properly before lighting a fire.