This is happennig because Salesforce has disabled TLS 1.0 encryption.
After June 25, 2016, at 9:30 AM PDT (16:30 UTC), all sandbox orgs -- whether existing, refreshed, or new -- will have TLS 1.0 automatically disabled and will require TLS 1.1 or later in HTTPS connections to or from the sandbox org.
For more information on Salesforce disabling TLS 1.0 refer - https://help.salesforce.com/apex/HTViewSolution?id=000221207
To fix this issue in Eclipse -
1) Update your eclipse.ini file to include this line: -Dhttps.protocols=TLSv1.1,TLSv1.2
You will find eclipse.ini file in eclipse installation directory.
2) If you are using Java 7, then you need to disable TLS1.0 and enable TLS1.1 and TLS1.2. (GO to control panel --> Java --> Advanced --> Advanced Security settings). TLS 1.0 is disabled by default in Java 8.
For Data Loader -
1)Uninstall older version of Data Loader and install Spring '16 version of Data Loader.
2) Install Java JRE 1.8.
Note:- Salesforce is going to disable TLS1.0 protocol on Production orgs in March 2017. So before salesforce disables TLS1.0, all the channels which are communicating to salesforce need to use TLS version 1.1 or higher.