I was using grease pencil just fine, but then after I locked a layer(and selected another) I couldn't draw because my drawing strokes would instantly disappear. I've checked opacity, visibility, frames, and X-ray. I just don't know what to do! If you would like images or a video or the file feel free to ask.
If you didn't nudge the frame and created by mistake another keyframe (i.e. when you don't draw on the desired keyframe and draw on an empty space on the timeline), then delete your Time Offset modifier, the blue wrench icon under Editor Type on the right, and your keyframes will be back to normal and you will be able to edit them.