We've been getting a number of inquiries on this forum from Plesk users who are having trouble getting certificates using Plesk's Let's Encrypt support.
If Let's Encrypt was previously working for you until May to June 2017 and then suddenly stopped working, the most common problem is related to IPv6. In late May 2017, Let's Encrypt started validating over IPv6 in preference to IPv4, for sites that offer both. Your site offers IPv6 if your hosting provider assigned you an IPv6 address and your DNS records include an AAAA record for your site. However, some Plesk (and other) users have found that their sites weren't set up properly to respond to IPv6 connections. If your Plesk integration worked before and recently stopped working, this is a good thing to check first.
For all other Plesk problems: You are welcome to ask questions on this forum. If your problem isn't something comparatively straightforward or familiar, this forum is relatively unlikely to be able to help you. So far, no Plesk developers have been participating here, and we can't easily debug issues with Plesk. Right now, most Plesk users can probably get better assistance on the Plesk forum, which has more active participation from the Plesk community.
You can find existing Let's Encrypt-related topics there via
Thanks for using Let's Encrypt.