Download a fresh gnutella.net host file from https://sabercathost.com/folder/117312/gnut_file and copy-paste the file into C:\Users\UserAccountName\AppData\Roaming\WireShare
Keep a copy of the file in case you need it over the next 12 months. After that if needed, download a fresh copy as it should have been updated by then.

Did you not use WireShare for a month or more?
I can't remember which WSHR version but bigjx decided to re-add the code to delete the gnutella.net file if WSHR has not been used for 28 or more days. I don't agree with his approach but that's it. His idea is that WSHR would then download hosts from web cache to create a fresh file but unfortunately the main webcache WSHR uses has been misbehaving over the past year.

Since WSHR last connected for you, have you installed or updated any security softwares? One of them might be blocking the connections.

@Lord of the Rings

First, thank you for your immediate and helpful response.

I have been using WireShare since version 5.6.3.1 (which was released on 2014-04-19) and for almost 5 years I have not had problems with connection. I usually start WireShare after I start my PC so it is always running. Some weeks ago I noticed that it failed to connect. I waited for some time to see whether the problem would disappear but it didn't. Before I posted this I had browsed through this forum and downloaded some LimeWire fixer- it turned out that it just extracted gnutella.net file to AppData. Last night I put that file to WireShare folder (AppData) but it did not help. So I followed your advice and downloaded a fresh copy of gnutella.net, copied it the WireShare folder in AppData but WireShare still cannot connect. Meanwhile I tested Shareaza and gtk-gnutella- they both were able to connect but after some time Shareaza lost Gnutella1 connection being connected only to Gnutella2. Is there anything else I should do to solve connectivity problems with WireShare? Any idea how to force Shareaza to connect to Gnutella1? All the time I see connecting status for Gnutella1.

It turned out that connection to Gnutella1 in Shareaza is shaky, sometimes it is lost, then the connection is restored.

Edit
I disabled all Windows updates about a year ago. I also disabled my Antivirus and then started WireShare bit it failed to connect again.

Off-topic 1
do you know whether WireShare and Shareaza have been abandoned for good just like Cabos, Gnucleus and Phex?

Off-topic 2
For me the most solid and robust file sharing program I have ever used is eMule- for more than 9 years version 0.50a has been performing flawlessly- not a single crash, settings never lost, no connection problems. It is a pity that eMule has been abandoned.