My site crashed on launch day.
I'm running Joomla 2.5 with Community Builder, K2,

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/k2.php on line 13

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/helpers/utilities.php on line 326

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/controllers/itemlist.php on line 35

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/helpers/utilities.php on line 326

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/helpers/permissions.php on line 242

Notice: Use of undefined constant K2_JVERSION - assumed 'K2_JVERSION' in /home/my-username/public_html/mywebsite.com/components/com_k2/helpers/permissions.php on line 252

Fatal error: Class 'K2Table' not found in /home/taichi9/public_html/mywebsite.com/administrator/components/com_k2/tables/k2category.php on line 14

Should I try re-installing K2?

backup first!
If you have a previous working backup try restoring it.
Installing k2 over the top is ok, but Do Not uninstall k2 unless you want your k2 tables to be deleted