sometimes it can take quite a while.. if after like 15-20min you still can't connect, you may ofc delete the blockchain and do a full resync... but in general, start it and just let it do its thing. most probably, it was a unclean shutdown and just needs a bit more time..

link? no, we just got ada ? do you have a cardano wallet? https://daedaluswallet.io/ or https://yoroi-wallet.com/ from there its just a few clicks, you can't do much wrong

no, pool might just stop paying rewards. but your coins are never at risk.. they never leave your wallet. delegation is just some information stored in the blockchain. so you can just re-delegate to an other pool (whenever you want).