'I accidentally closed a web page I still need!'
1. No problem: Right-click on any of the tabs at the top, from the drop-down select; 'Reopen closed tab' or 'Recently closed tabs'.
"I avoid rebooting my computer every day, because I don't want to close all my active windows. (But then my computer runs slower and slower.)"
2. Try a new setting instead: From the Internet Explorer Menu bar, go to 'Tools', Internet Options, on the 'General' tab (about mid-way down) you will see the 'Startup' section. Under it click the option for 'Start with tabs from the last session'.
(For more ideas on working with IE tab features here is a good article: https://www.winhelp.us/internet-explorer-10-tabs.html)