My current visa is valid until December
Nope, your visa became invalid the moment they put CWOP stamp on it. It means Cancelled without prejudice.
Cancelled Without Prejudice:
A stamp a U.S. Embassy or Consulate puts on a visa when there is a mistake in the visa or the visa is a duplicate visa (two of the same kind). It does not affect the validity of other visas in the passport. It does not mean that the passport holder will not get another visa.
U.S. Visas Glossary
Prejudice or not, the visa stands cancelled. If you also have another valid US visa, that other visa is still good to use, but the one they put the stamp on is no longer valid.
I am scheduled to travel back to the States in two days
Please do not travel back to United States if that was the only visa and you are a visa national, You will not be allowed to board the aircraft and if you are allowed you will certainly be refused entry into the US.
I understand that it is very unfortunate and it is stressful to be suddenly left without a visa 2 days before you are about to go back but it is what it is, any advice telling you to try your luck will be a disaster and if you don't cancel your ticket in time you risk losing money as well.
My Advice would be to cancel your flight and follow the instructions provided by the consulate in the refusal letter.
What does a visa denial under INA section 221(g) mean?
A visa denial under section 221(g) of the INA means that the consular
officer did not have all of the information required to determine if
you are eligible to receive a visa. This means you are not eligible
for the visa now, but your case is pending further action for one of
the following reasons:
Your application is incomplete and/or further documentation is
required - Applicants whose application forms or other documentation
are incomplete are refused. If further documents are required to
complete your case, you will be informed what is needed and how to
provide it to the embassy or consulate. You will also be given a
letter stating your application has been denied under 221(g) and
listing which documents you need to provide.
Administrative processing - Further administrative processing of your application is required before a decision can be made regarding your eligibility for a visa.
You will be given a letter stating this and next-step instructions
after the administrative processing is complete.
Is there something I can do about a refusal under section 221(g)?
If your application was denied because documentation or information is
missing, you can provide the missing documents or information as soon
as possible. After submitting the documentation, your visa application
can then be processed to conclusion to determine whether you qualify
for a visa. You have one year from the date you were refused a visa to
submit the additional information. Otherwise, if you do not provide
the required additional information within one year, you must reapply
for the visa and pay another application fee.
If your application requires further administrative processing, this
takes additional time after your interview. Processing times can vary
based on individual circumstances.
Source: State Department