Oracle Error: the sga requires more space than was allocated for it

(Oracle SQL 12c running on Windows 10)

My installation has previously been working fine. I was trying to set up one of my tables to use the In-memory option available in 12c. I was following this guide: https://blogs.oracle.com/In-Memory/entry/getting_started_with_oracle_database
I wanted to allocate 4Gigs of system RAM to be used as in-memory table storage space for a table I would specify later. To do this I ran these commands using the guide as a reference:

This one to allocate 4G of RAM space:

ALTER SYSTEM SET inmemory_size = 4G scope=spfile;

And this one, which is what caused the problem:

ALTER SYSTEM SET sga_target = 75G scope=spfile;

The guide said "By default, the installer set the SGA_TARGET to 71G, " which is where I came up with 71G + 4G = 75G.

Now of course I get the error when trying to connect to the DB:

ORA-27101: shared memory realm does not exist

the sga requires more space than was allocated for it

Since I can't connect to the database because it won't start, how would I fix the invalid setting? Is there some sort of "undo"?

Edit:
No, there is no backup of the spfile or pfile that I can use to generate the spfile. I have manually changed the sga_target to small values that should be fine but still get the error no matter what. I also tried creating a new, additional database. When I did that, the other database started working for some reason. It just looks like the new one though and is missing new tables I had previously created.