Great....I've burnt the lawn with weedkiller...what do I do?

[SpinningBirdKick]

Gardening virgin here, and please don't judge too much!

I was given some weedkiller which I was told would be OK for the lawn, to kill the dandelions that I've got growing in earnest.
So, as you do, I merrily went along spraying them, left them overnight and came downstairs to find the lawn has MASSIVE scorch marks on it- it's killed big portions of grass as well as weeds.
After questioning my weedkiller source- I found that I'd been given resolve in an unmarked bottle- which clearly states NOT to use on lawns (for reasons above).......

Now this portion of the lawn is right back at the garden, so it's not visible from the windows, but please can anyone give me advice?

Old Ron from next door says that it will grow back next year, as the weedkiller is biodegradable, or should I get some grass seed down.

Any advice is appreciated- It's my first garden and this is (so far) my only disaster- and I really want to get it back to life

resolve is a systematic weed killer so it will travel down and kill the roots as well at the top layer of grass. It doesn't remain active in the soil so you will be fine to scatter fresh lawn seed then add a thin layer of compost and keep mist until the seeds germinate.

A tip from my dad
If you have any sand then it is a good idea to mix the seed with damp sand ( a 50/50 mix) the day before you want to spread it. The moist sand will help with germination and help the seed to stay in place.