Twitter’s API v1 is retired.
https://dev.twitter.com/blog/api-v1-retirement-final-dates
There is a new API available but if you are using any WordPress plugins that utilize twitter, these may have ceased to work if the developers haven’t adapted to the new API.
If you’re looking for a good replacement to a plugin for showing you twitter feed on your website, check out: https://twitter.com/settings/widgets/new