Hi all,
I'm replying because I have the same setup as the original poster: ESXi 5.1 hosts with a Synology, using iSCSI.
This is not an elegant answer--and should certainly not be used in a production environment--(but since it's a Synology, I'm guessing this situation is a home lab scenario like mine is).
I had an ESXi 5.1 host crash hard, no configs changed on it at all, but when I brought it back online it would not reconnect to the Synology iSCSI target. I tried changing network configurations on the host, different bindings, removed and readded the software iSCSI adapter...nothing worked.
Finally, after determining that it was not a configuration issue--nor had any hardware gone bad, (thankfully!), I powered off the Synology, waited a minute, and brought it back online. When it was back online, rescanning the storage adapters on each host made them see the iSCSI storage again!
I couldn't say for sure why this happened in the first place, but I suspect an outdated DSM version on the Synology. I'm running DSM 4.1-2668, and the latest version as of this post is 4.3-3827. Hopefully, the update is a more permanent fix or preventative measure.
HTH!