Don't panic, it can be fixed. Another option is to pull the dropped stitch to the back and secure it with a piece of yarn by running it through the stitch and then weaving in the ends. Do count and see if your count is off. I sometimes actually slip a stitch and leave a yarnover on the needle and then without noticing knit the yarnover. You may have done that, I can't tell by looking. If you did your stitch count will be correct. If it is a true dropped stitch and you don't manage to hook it back to up to the needle, you'll need to increase one to make up for it.
ETA This is stockinette. It doesn't matter whether it was on the knit side or the purl side. Your fix will be done on the knit side and all stitches will be knits for the repair. HTH as it's one less thing to be concerned about.
Looking again, I think it really is a dropped stitch.