Washing machine door wont open... help ?

Hi

Our washing machine has locked for the second time in about a week, its a refurbished washing machine (hotpoint) only had it a month ,worked fine the first few washes but one day last week we did a wash and it cut out about a minute into the wash, finally got it working and did the wash but then the door wouldnt open, i switched off by the mains, did another wash, still locked etc. We got the company we brought it from to come up and sort it, he managed to open door and tightened up the washing machine door lock or something, ive done two washes since and everythings been fine, i done a wash tonight , had trouble shutting the door to start with ,door wouldnt lock or hold, when it did nothing happen, machine wasnt starting up, waited, still nothing, so i opened the door again and closed it and it finally started up ,now its locked again and wont open, ive switched the machine on and off, ive put it on a quick spin wash, still the same. what could the problem be? happened twice now not happy