I had an issue where every time I put my bike in gear 3, it would just keep "skipping". By this I mean that it wouldn't stay in gear 3; the chain would go onto it, then fall onto gear 4, then go back onto it, then back on 4.
Some Googling showed me that I might need to adjust the rear derailleur. So I set about doing this today by first putting my bike onto the biggest cog and adjusting it so the derailleur was in line with the cog. Then I shifted it down to the smallest one and did the same thing (I think I got a bit confused between this and changing the screws to line these two up with something).
Anyway, as luck would have it, this fixed the problem of the skipping however I now can't shift into gears one or two (the two biggest cogs). Like I literally can't flick down the gear lever to put it into these gears.
What could be the issue? I would love some advice.