The path to your license file is set incorrectly. The default value of the license file setting in config is:
<setting name="LicenseFile" value="$(dataFolder)/license.xml" />
If you have not change this, then most likely the issue is the
dataFolder variable is not set correctly. Use a patch config set to update it to match the folder path your website is deployed to:
The reason for the failure in your upgrade, at a guess, is that the
dataFolder variable was updated directly in the
<sitecore> section of web.config before, and when you upgrade to Sitecore 8.1 this has been moved to separate /App_Config/Sitecore.config file and therefore you lost your old setting. It's always best to use config patches to override Sitecore settings.