Can I import my old version content into a new version 2.33?

The above link is your answer. Looking for on community also. Discussed already.

You cannot directly upgrade from 1.8 to 2.3. You would need to upgrade step by step from 1.8 to latest 1.9 version, then to latest 1.10, 1.11 and then 1.12. Only from the latest 1.12 version you can directly upgrade to the latest 2.3 version.

If the fatal error is due to php 7, then you need to downgrade to php 5.6 again. Check the server error log what it tells you about the reason for the fatal error. If the error started to appear after the upgrade to php 7 it's a likely reason though. Maybe the php mysql extension was removed when upgrading to php 7 only. Then Elgg can't connect to the database. If that's causing the fatal error it might help to add the mysql extension again. Then you might be able to upgrade the Elgg version at least. Though I can't tell if this works. Only Elgg 2.3 will be fully compatible with php 7. With any older version of Elgg you will have issues (and it might not even work at all).

Best would be in any case to downgrade to php 5.6 again and then do the upgrade of Elgg and 3rd party plugins. Only afterwards you could upgrade to php 7 again. Keep also in mind that Elgg 1.x versions require the mysql php extension while Elgg 2.x requires the pdo php extension to connect to the database. Best would be to have both installed.

If you can't downgrade php for whatever reason, you could still fix the upgrade issue for example on a local server installation that still runs on php 5 by using your current Elgg installation as starting point (install folder+database+data folder) and then upgrade to 2.3 step by step and then migrate the updated installation again to the server still running on php 7. See http://learn.elgg.org/en/stable/admin/duplicate-installation.html.