Was it a bus from the aircraft to the building?

You may have a few problems if you are on the rolling 90 day limit for Schengen visitors from Third Countries. If you have a Schengen or Irish passport you don't need to worry, but for the time being - the next 181 days to be more precise - I would retain proof of arrival, such as your boarding pass or luggage label. Personally I would not seek out a police officer since I know what the official advice is going to be: go to the offices of SEF Lisbon, wait a long time, explain the situation and then get a stamp, begrudgingly. It happens all the time, only the Swiss, Germans and Dutch are bothered by it. If your other and future visits means that you don't go over 90 days with the missed stamp then you don't need to worry either.

It happens more in Spain than anywhere else, the Guarda are not paid extra for late shifts and weekends so if the Copa del Rey is on you'll not get a stamp inbound.