1. backup your wallet and conf files.
1b. backup your wallet and conf files again.

2. try adding these nodes to your conf file;

addnode=162.243.222.146
addnode=198.199.116.77
addnode=162.243.205.200
addnode=162.243.49.230
addnode=162.243.222.146
addnode=198.199.116.77
addnode=162.243.205.200
addnode=162.243.49.230
addnode=72.187.27.243:24242
addnode=188.178.145.136:24242
addnode=85.238.96.186:24242
addnode=24.212.252.61:24242

if no luck, then you may have to

3. delete the blockchain files (NOT your wallet.dat or .conf)