I've just had a look on Smartwings' web site and it would appear that the Friday flight is still running (and selling at 80 Euros per person one way) but that the Monday one isn't. So - one option might be to fly Smartwings one way and come back some other way. In the circumstances I understand that this would not necessarily be an option that you would want to take!
I looked at Easyjet who do fly to PRG but not from AMS. The nearest they fly from is Dortmund (DTM) near the German-Dutch border, and they have some good deals for the days you mention - but there is no way of getting there from AMS (by train) in time for the outbound flight on Friday. Easyjet to DTM might be a possibility for the return on Monday, though.
Another thought was that maybe you could somehow not take the LGW-AMS part of your outbound flight and make your way direct to PRG from one of the London airports. Probably not a good idea as it could result in your return reservation being cancelled... also, I couldn't see any cheap deals from LON for that Friday afternoon. Friday is a popular travel day at the best of times (with people going away on week-end breaks), and with this being Valentine week-end I suspect more people will be travelling than usual.
I'm afraid you may be left with no choice but to pay considerably more than you had planned to pay.... and with it being Valentine week-end I would recommend you book an alternative flight as soon as you possibly can before fares go up even more.