Here are the steps for how to clear your cache on different web browsers. Once you clear your cache, you should be able to log into Sendspark again.
Edge:
Click "Remove all shown"
Google Chrome
Click "Delete displayed data"
Firefox
Go to
about:preferences#privacy
Head to "Cookies and Site Data" section
Click "Manage Data"
Search for sendspark.com
Click "Remove all shown"
Safari
Select Preferences under the Safari menu.
Select the Privacy tab.
Select Manage Website Data.
Search for sendspark.com
Click "Remove all"
After you've cleared your cache, if you STILL can't log in, you can try reseting your password or logging in via Google authentication.