With a free DonationMatch account, once you reach your limit for approved requests (per event), any pending requests are put on hold, which means donor companies can no longer see or respond to them. As soon as you upgrade your event or account, the requests will automatically be reinstated and show up to companies as if nothing happened.
If companies wish to donate to your event on their own, they will be able to find your event and send you an offer, regardless of whether your event is upgraded or not. Companies who wish to send offers can choose to do so, and you will be able to accept any offers you wish.