If subdomains work and the main domain doesn't, this is likely to be due to DNS propagation.
If your address has been cached, you will need to wait for your DNS TTL to expire before the address is cached again - your TTL is currently set at 14400 seconds (or 4 hours).
Querying the system default DNS server (in my case, Google's public servers):
cwatson@thor:~$ nslookup markupchop.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
Name: markupchop.com
Address: 93.188.160.57
Querying your nameserver:
cwatson@thor:~$ nslookup markupchop.com 31.170.163.241
Server: 31.170.163.241
Address: 31.170.163.241#53
Name: markupchop.com
Address: 93.188.160.57
Get your nameservers:
cwatson@thor:~$ dig ns markupchop.com
; <<>> DiG 9.8.1-P1 <<>> ns markupchop.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16615
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;markupchop.com. IN NS
;; ANSWER SECTION:
markupchop.com. 14399 IN NS ns3.markupchop.com.
markupchop.com. 14399 IN NS ns2.markupchop.com.
markupchop.com. 14399 IN NS ns4.markupchop.com.
markupchop.com. 14399 IN NS ns1.markupchop.com.
;; Query time: 35 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Fri Jul 25 17:08:55 2014
;; MSG SIZE rcvd: 104