DIAGNOSIS That 'L' stands for 'Keep Warm' mode. The pot has finished its pressure cook time and has automatically switched to keeping your food warm. The number is just the count of how many minutes it's been in that mode. It's not broken, it's a feature.
FIX Nothing to fix! This is normal operation. When you're ready to open it, just press 'Cancel' to turn off the Keep Warm function. Then, you can either do a Quick Pressure Release by moving the steam valve to Venting, or let the pressure come down naturally (Natural Release). Once the float valve drops, it's safe to open.
IF THAT DIDN'T WORK If you pressed 'Cancel' but the display is still on or unresponsive, do a full power reset. Unplug the cooker from the wall, wait 30 seconds, and plug it back in. This clears any temporary electronic hiccup.
HEADS UP The 'L' on the display confuses a lot of folks—they think it's an error code. It's not. If you don't want the Keep Warm mode to kick in at all, you can turn it off before you start cooking. After you set your time, just press the 'Keep Warm' button until the light above it turns off.