If your seedlings are keeling over, then it's probably damping off that's the cause.
The stems go transparent, then collapse - nothing you can do will make a difference at this point - the seedlings are dying or already dead.
Most likely, it will happen even before the 'true leaves' have formed.
This is a fungal problem and it could be caused by your trays, soil, overwatering or overcrowding.
It is encouraged by humid conditions and overwatering.
It's quite common in glasshouses - the warmer temperatures encourage the fungi.
You might find it happens if you're sowing indoors either in preparation for planting out or sowing herbs for indoor winter use.
It very rarely happens in outdoor sowing.
It's so disappointing - you wait patiently to see the first green shoots emerge - care tenderly for them and your reward is a sad, flaccid seedling that just expires on you.
Once the sowing begins to fail, there's nothing you can do about it - you can try removing the diseased seedlings, but chances are, the rest of them will fail too as the same conditions apply and you might as well get rid of the whole lot and start again.
At this point, you need to either sterilise the compost or dispose of it and use fresh pots, compost and seed.
If you're sowing indoors, then there's not a lot you can do - not many variables here really - different pots, new compost - but you'll likely find that the problem is overcrowding, overwatering, humidity or a combination of all three.
It's vital that your pots are clean and the soil is sterile.
The easiest way to combat it is to use a copper fungicide solution.
You decide whether you want your seedlings to mature into plants or use copper - I personally prefer to buy a tin of Cheshunt Compound - look for 'copper fungicide' on the tin if you don't live in the UK. It's no longer available in the UK sadly - it's been removed from the market.
I don't consider this to be 'chemical' intervention - copper is a natural element in my book and I don't have a problem with using it.
Don't inhale it though - it makes your eyes smart and takes your breath away.
Read the instructions on the tin carefully - I only ever make up a pint (0.5 litre) at a time.
A tin will last me several seasons of growing. I throw it away when it begins to rust - that's how long it lasts.
I use a plastic spray bottle to keep the made up formula in and water my seedlings with it.
It is very effective in preventing damping off.
It can't be cured - it's happened, the seedlings are dead and there's nothing you can do to save them.
You can prevent it by exercising 'perfect' growing conditions - something few of us are able to do - or by using the copper solution as instructed on the tin.
These page will also be of interest to youThe Herb Guide › Growers Dictionary › Damping Off