Inventory running low

Hi, I’m still fairly new to Amazon. Since a few weeks back I been getting a few sales a day. I checked my inventory and according to that I send out some more product for a refill. I’m just getting anxious because I’m checking the ups tracking and it says it’ll get to the warehouse longer than I anticipated, not to mention Amazon processing times also! What is going to happen if I run out of inventory before it’s refilled?! Thanks in advance everyone.