A lawful permanent resident unable to return to the US, may encounter one of these scenarios:
If you are outside the US longer than 1 year and you obtained a re-entry permit, you can enter the US for up to two years. You can only apply for a reentry permit from within the US.
If you are outside the US longer than 1 year and you did not obtain a re-entry permit before leaving the US, the actual green card becomes invalid for reentry and you will likely need a returning resident visa. You will need to prove to the consular officer that you have not abandoned your residency.
Due to COVID-19, the consulate may not have reopened or you may have been denied the returning resident visa. If your green card is still valid, you can travel to the US and tell the CBP officer that you did not abandon your residency. The officer can, at their discretion, waive you into the US, may ask you to complete Form I-193, or may allow you in but issue a Notice to Appear before an Immigration Judge in removal proceedings.
If you have been outside the US for more than 180 days, when you try to reenter the US, you will be considered an applicant for admission, be subject to more scrutiny, but may not necessarily be denied entry especially if the delay was due to COVID-19 travel restrictions. Further, the CBP officer will look into other grounds of inadmissibility, in essence looking for reasons to deny you entry.
If you are a conditional resident, most of these will apply to you as well. However, if you are a conditional resident who already applied for the removal of conditions, the situation could become significantly more difficult depending on the timeline.
Extensive travel abroad regardless of the reason will also affect eligibility for naturalization. There are two issues that come into play about travel when applying for naturalization: physical presence and continuous residence.
Except in certain categories, an applicant for naturalization must have two and a half years of physical presence in the 5 years prior to filing as each day spent outside the US counts against the days accumulated in the US. Further, if you are stuck outside for over a year, then you will need to wait at least four years and a day before you apply for naturalization, and the counting starts again for both physical presence and continuous residence.
In addition, the applicant who has been outside the US longer than 180 days is considered to have broken the continuous residency, but this can be rebutted through evidence. There is no current way under the rules to not count the days abroad if the circumstances were beyond your control.
You should save yourself time, money, and aggravation by consulting with an experienced immigration attorney who can help you navigate these issues and devise an appropriate plan.