I've read the blog posts koga305 mentions and they are good for a first time DM.
There's a lot going on in Lost Mines and it's easy for thing to go awry.
Just tonight I was running LMOP for Adventurers League and they were finishing up the Red Brand Hideout after clearing half last week.
Unfortunately only one of the three players was back from last week and there were two new first level characters along with the second level paladin in a dungeon designed for four second level characters.
There a room with three bugbears which I reduced to one. Even knowing the bugbear was there they lost and all three PCs dropped to zero HP. Luckily the dungeon has a jail so I put them there with one hp each and one guard. Nothing like no equipment and one hp to get the players thinking.
By good fortune, they made their way immediately to the armory, though they didn't know it was there. They thought going past the crypts was a bad idea (correct). They found all their stuff and rested.
In any event, be prepared for stuff like this to happen.