Depends a bit on the pick, but generally you should try to get some jungle farm if your jungler is on the other side of the map(i really often take gromp/krugs as toplaner in a bad matchup), else try to atleast stay in xp range till you have all needed skills to gank(on most champs, that's lvl 3, on akali it would be 6)
Once you have the needed level, try to roam to other lanes, invade the jungle together with your jungler, try to get some gold via kills/assists and objectives. If your opponent builts up a big wave, call for the help of your jungler, not for a real gank atampt but to provid savety while you farm the wave.
This all is asumeing you really struggle hard to get any cs without risking to die.
Without initial mistakes/jungle-impact, nearly no matchup is that bad that you can't farm somewhat savely underneath your turret. Yes, you will drop alot cs, but generall you usually can keep up in xp and hold the rower till your get some gear and levels. And always watch up for opportunities to impact other lanes. Even with a horrud farm, if your not totally underleveled, a numbers advantage will always be a big deal, helping to swing around botlane fights or help your jungler to fight of the opposing one. Just be carefull with your lane-opponents movements and don't commit to fights if you know he'll join since he obviously is stronger at than you and will have more impact on the fight at that time.
Calling for aid from your jungler sure can help aswell, provided your not to far behind already which would risk a double kill for your opponent.
Also, don't rely on your jungler, he might have other things on his mind, the map has 3 lanes, 4 jungle parts and some objectives after all, and ganking a loosing lane often is not worth the effort as the chances to get a kill are rather low, depending on how hard it is lost and how the matchup looks like.
Also, if you already fed like 3-4 kills somehow, just kepp farming as much as you can and oushing out waves, try to buy as much time as you can should they try to all in you. With 4+ deaths, your barly worth more than a minion wave, at that point tradeing a few waves for your live kinda is worth it, + the shutdown money of your opponent increases.
This obviously is a emergency solution, idealy you play safe enough to not reach that point, but once it happens, it's good to keep in mind that death strikes reduce your gold value alot. Also, try to not get assists till the laneingphase is over in that case, as asissts reset your gold value.
Also, adjust your builts to be more cost efficient, focuseing purely on what your kob is in a teamfight. This means you go full tank on frontline champs with maybe ZZ-rot to counter push better, and it means either utillity builts or full damage builts for the rest.
With utillity i mean something like frozenmallet on ranged ad to help peeling, or black cleacer to shred armor for your team.
The only other option is full damage with not even one though wasted on def, just trade your life for that off an opposing one. That is an all or nothing approach, but desperate times require desperate messures.
Again, this applies only if you fall behind REALLY hard.