This is a common thing for us guitar players, Maybe some of you guys can share a better, effective and more efficient tips than I can, but here's what I do, when ideas doesn't flow well, I usually :
1. Copy a guitar riff or solo from an album that interests me (or from a YouTuber) then I add my own twist to it.
2. I rest for a couple of days, doing movie marathon and not listening to any guitar-oriented music
3. Tumutugtog ng song-hits using my acoustic guitar 
4. Lift weights
if strictly writing guitar lines, I find that this works well for me, and is especially helpful for rock/pop/funk bass too:
1. pick four notes. Only four notes, no transpositions, no octaves, just those four notes.
2. set a metronome to 120bpm
3. plug in to a recorder and start rolling
4. keep on playing anything with those four notes and don't stop until you're tired of doing the exercise. note that there's no fixed style or key so if you suddenly play into a different groove or time sig just keep doing it again and again as long as it keeps to the click. there's usually three to five really groovy riffs in there.
if it's lyrics wise, I just write about food then change the food-related nouns and pronouns into people. trust me, you can replace any song's contents to make it talk about food, so it means you can do the reverse to aid your songwriting.
