Solution: give it 3-5 minutes. If the worksheet chart used to work on SharePoint webpart before and if you did not modify the data connection(s), try again later. It seems to take a while for SharePoint to handle the updated Excel files in the back-end.
If the problem persists...
1. Check if Excel Services is running.
Central Administration -> System Settings -> Manage Services on Server -> Excel Calculation Service -> Check if "Started"
2. Check if Excel Service Application is associated to Web Application in question.
Central Administration -> Application Management -> Manage Web Applications -> Select Web Application -> Configure Service Associations
3. Check if Trusted File Location is configured for Excel Service Application.
Central Administration -> Application Management -> Manage Service Applications -> Excel Service Application -> Trusted file location -> Add trusted file location -> Add address as https:// and Location Type as Microsoft SharePoint Foundation -> Keep the rest as is
4. Check if document library configured to open documents in browser.
Site -> Library -> Library Settings -> Advanced Settings -> Check for "Opening Documents in the Browser" Use either Server Default(Open in Browser) or Open in the Browser.
5. Grant "Excel Service Account" access to SharePoint Content database via PowerShell.
$wp = Get-SPWebApplication -identity
$wp.GrantAccessToProcessIdentity("Domain\Account that runs Excel Service")
**To check for service account - Central Administration -> Security -> Configure service accounts
6. Check if SharePoint Web Services in IIS has ASP.NET Impersonation disabled.
Select SharePoint Web Services from list of available of sites. -> Select Authentication from right panel and available authentications are displayed. -> Disable ASP.NET Impersonation if enabled.
