Okay, this is weird...
I had trouble shifting into 3rd gear from two, but I could shift into 4th and then back down to 3rd.
Later on, I found that I couldn't shift into 7th at all.
Upon closer examination, I found that nothing was frozen into place or anything, but after some Googling and YouTubeing, I figured I'd adjust the cable attached to the handlebars. But now, my bike does shift into proper gear -- SORT OF...but it's one off. Basically, 1st and 2nd are the same, and no shifting until 3rd gear, and judging by the resistance and how the chain moves, it appears to actually be in 2nd when I shift to 3rd, 3rd when I shift to 4th, etc.
After this, though, no amount of adjusting the cable one way or another makes a difference.
Is there an easy way to fix this, or should I just bring the bike to the guys at Village Cycle Center? One of the techs there told me, "Google is your friend!" when I brought my bike there for some adjustments or repairs once. And of course, when I do the YouTube thing, I always end up making my bike worse...
In case these deets are important, it's a 2014 Trek Shift 2 hybrid.
You likely need to adjust the indexing on your rear derailleur. Random adjusting of the cables won't solve the problem now. Here's a link to a good park tools article and video on how to adjust rear derailleurs. http://www.parktool.com/blog/repair-help/rear-derailleur-adjustment
It can be frustrating the first few times you do it, but if you take time and figure it out, you've picked up a repair skill that solves one of the more common problems people face in terms of poor shifting. Also, if your shift cable is old and/or worn, it's often worth replacing it before you spend the time fixing the adjustment issue.
A LBS will do the adjustment for probably around $15 to $20 if you want to avoid the time and frustration.
+1 on the Park Tools article. Sounds like you have insufficient tension on your cable. Derailleurs can be a major PITA sometimes.