Centos – Don’t work yum update (yum doesn’t have enough cached data to continue)

I have installed things successful via yum on centos 7. Then I install epel repo (yum install epel-release ) and always give me followings error and after that I couldn't install anything

 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

 1. Contact the upstream for the repository and get them to fix the problem.

 2. Reconfigure the baseurl/etc. for the repository, to point to a working
    upstream. This is most often useful if you are using a newer
    distribution release than is supported by the repository (and the
    packages for the previous distribution release still work).

 3. Run the command with the repository temporarily disabled
        yum --disablerepo=<repoid> ...

 4. Disable the repository permanently, so yum won't use it by default. Yum
    will then just ignore the repository until you permanently enable it
    again or use --enablerepo for temporary usage:

        yum-config-manager --disable <repoid>
    or
        subscription-manager repos --disable=<repoid>

 5. Configure the failing repository to be skipped, if it is unavailable.
    Note that yum will try to contact the repo. when it runs most commands,
    so will have to try and fail each time (and thus. yum will be be much
    slower). If it is a very temporary problem though, this is often a nice
    compromise:

        yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

I have searched a lot on the internet but I couldn't find solution. Some people maybe from proxy. even if I added my proxy information to yum.conf, but again I failed.

Then I want to check that If I delete epel repo then probably It may works and I deleted epel-repo from system after that I could install.

Please help me I spent this issues two days